Self-copying papers or carbonless copying papers have been known since the early '50s. They are now used in large amounts in banking and freight forms, bills of lading, invoices, etc. They usually consist of two or more sheets of paper, one on top of the other, in which the upper sheet has a chromophore or color donor layer on the back (CB=coated back) and the lower sheet has a dye or color acceptance layer on the front (CF=coated front). The main component of the color donor layer is thick-walled microcapsules of gelatins, polyurethane, melamine-formaldehyde and similar substances that contain solutions of dyes in the so-called leucoform. These only slightly-colored dye precursors, mostly from the class of di- or triphenylmethanes, thiazines, spiropyrans, or fluorans, act as electron donors (Lewis bases) and can be converted to the dye form in a chemical reaction with electron acceptors (Lewis acids). Such Lewis acids are found in the dye acceptance layer in the form of acid phenolic resins, zinc salicylates or acid-activated clay minerals, especially in the form of acid-activated smectitic layered silicates. If the walls of the microcapsules are destroyed during writing on the carbonless copying paper by the writing pressure, the capsule contents (the dye solution) is released and develops on the Lewis acid acceptance layer, forming a copy.
The individual color developer systems have different reaction mechanisms. Thus, the organic products (phenolic resin, zinc salicylate) are partially dissolved by the capsule solvent and the color reaction occurs in a homogeneous, organic phase. The copies so developed exhibit good color stability, even at high humidity (tropical climate) and limited sensitivity to environmental effects (e.g., NO.sub.x, SO.sub.2, etc). The relatively slow development of the copies (especially at low temperatures) and severe yellowing of the paper under the influence of light are considered negative, mostly in the phenolic resins.
The acid-activated clays (smectitic layered silicates) are capable of absorbing the dyes and developing them in heterogeneous catalysis, because of their high specific surface areas of about 300 m.sup.2 /g. The capsule solvents are absorbed, in this case, in the pores of the clay. Rapid color development, good light stability and limited yellowing tendency are advantages in the acid-activated clays. On the other hand, a problem is the sensitivity of the copy to high humidity and environmental gases.
Attempts to combine both systems (phenolic resin and acid-activated clay) have recently been observed on the market. Both compounds are combined during coating composition preparation and applied together to the base paper. The papers so produced, however, have a strong yellowing tendency and significant sensitivity to humidity.
It has now been found that the aforementioned problems can be eliminated and the quality of the color developer pigment significantly improved, if the phenolic resin is condensed on the surface of the acid-activated smectitic layered silicate.